Reflecting mirror and rear projection display making use thereof

ABSTRACT

There is provided a reflecting mirror for use in a rear projection display and so on, comprising a glass substrate at least one surface of which is polished to have a predetermined surface waviness (0.05 μm or less on the measurement condition that the cut-off value is 0.8 mm to 8 mm), and a reflecting film which is formed on the polished surface of the glass substrate by film-forming in order of aluminum (Al), magnesium fluoride (MgF 2 ) and titanium dioxide (TiO 2 ). As a result, it is possible to obtain a reflecting mirror wherein any pattern of light and shade in an image projected on a screen which occurs due to the difference in light intensity resultant from irregularities in the mirror are prevented, and thereby the image quality can be improved.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a reflecting mirror for use in adisplay device by which an image to be displayed is enlarged andreflected on a mirror and thereby is projected on a screen, such as thatof a rear projection television, and the invention relates to a rearprojection display making use thereof.

[0003] 2. Description of Prior Art

[0004] Conventionally, in a reflecting mirror for a rear projectiondisplay, a reflecting film of aluminum is formed by an evaporationmethod or a sputtering method. Also, a glass substrate which is producedby a float process is normally used.

[0005] However, on a surface of such glass substrate formed by a floatprocess, minute irregularities which are referred to asmicrocorrugations (around 0.1 μm-0.16 μm on the measurement conditionthat the cut-off value is 0.8 mm to 8 mm) occur parallel to the transferdirection of the float process in the course of the production. Suchirregularities inevitably occur on a surface of the glass substrate dueto the production process of the glass substrate.

[0006] Therefore, in a case of using a reflecting mirror which isobtained by forming a reflecting film on a surface of such glasssubstrate for a rear projection display, parallel light which isincident on a reflecting mirror 100 is reflected by a reflection surface101 which has irregularities, so as to be deviated from being parallel,as shown in FIG. 4. As a result, a pattern of light and shade occurs inan image projected on a screen 102 due to the spatial difference inlight intensity, thereby presenting a cause of remarkable deteriorationin image quality.

SUMMARY OF THE INVENTION

[0007] It is therefore an object of the present invention to provide areflecting mirror, wherein any pattern of light and shade in an imageprojected on a screen that may be due to the spatial difference in lightintensity is not present, thereby enabling an improvement in imagequality, and a rear projection display making use thereof.

[0008] According to one aspect of the present invention, there isprovided a reflecting mirror for use in a rear projection display and soon, comprising a glass substrate at least one surface of which ispolished to have a predetermined surface waviness, and a reflecting filmwhich is formed on the polished surface of the glass substrate.

[0009] According to another aspect of the present invention, in thereflecting mirror mentioned above, the glass substrate is produced by afloat process to have minute irregularities of 0.1 μm or less on themeasurement condition that the cut-off value is 0.8 mm to 8 mm.

[0010] According to still another aspect of the present invention, inthe reflecting mirror mentioned above, the predetermined surfacewaviness is 0.05 μm or less on the measurement condition that thecut-off value is 0.8 mm to 8 mm.

[0011] According to still another aspect of the present invention, inthe reflecting mirror mentioned above, the reflecting film is obtainedby film-forming in the order of aluminum (Al), magnesium fluoride (MgF₂)and titanium dioxide (TiO₂). Formed by this method, it is possible thatthe reflecting mirror has a reflectance of 94%, while a reflectingmirror formed only of aluminum has a lesser reflectance of 88-90%.

[0012] According to still another aspect of the present invention, thereis provided a rear projection display, comprising the reflecting mirrormentioned above, an optical unit, and a screen.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a cross-sectional view of a reflecting mirror accordingto the present invention;

[0014]FIG. 2 is a structural view of a rear projection display accordingto the present invention;

[0015]FIG. 3 is a diagram showing the results of measurement ofirregularities on the surface of the glass substrate before and afterpolishing; and

[0016]FIG. 4 is a view illustrating a diffusion of light reflected on areflection surface of a reflecting mirror and pattern of light and shadeoccurring on a screen in a case where a glass substrate hasirregularities on a surface thereof.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

[0017] Hereinafter, embodiments according to the present invention willbe fully explained with reference to the attached drawings.

[0018] In order to produce a reflecting mirror according to the presentinvention, a glass substrate, which is produced by a float process tohave a thickness of 3 mm, is cut to be a predetermined size, andthereafter the end surface thereof is chamfered. It is preferable thatminute irregularities on the surface of the glass substrate are equal toor less than 0.1 μm on the measurement condition that the cut-off valueis 0.8 mm to 8 mm, taking a surface waviness required for a subsequentpolishing process into consideration.

[0019] Next, with a plane polishing apparatus using cerium oxide as aabrasive material, the surface of the glass substrate is removed byaround 3 μm and minute irregularities thereon is reduced, and therebythe surface waviness thereof is made to be of a predetermined value.

[0020] It is preferable that the predetermined surface waviness is equalto or less than 0.05 μm on the measurement condition that the cut-offvalue is 0.8 mm to 8 mm measured, for example, using the SURFCOM 479Amanufactured by Tokyo Seimitsu Co., Ltd. as a surface roughnessmeasurement apparatus, taking an image quality required in a case wherethe reflecting mirror is applied as a mirror of a rear projectiondisplay into consideration.

[0021] However, if the predetermined surface waviness is equal to orless than 0.001 μm on the measurement condition that the cut-off valueis 0.8 mm to 8 mm, it takes a longer period of time to carry out thepolishing and the cost of producing such a reflecting mirrorconsequently increases.

[0022] Next, the polished glass substrate is immersed in alkalinecleaning agent including potassium hydroxide and is cleaned with anultrasonic washing machine. Thereafter, an ultrasonic cleaning with purewater is conducted upon the surface of the glass substrate, therebymaking the surface clean. However, as a method of cleaning with purewater, a well-known method such as a brush cleaning can be used as wellas the ultrasonic cleaning.

[0023] Next, as shown in FIG. 1, a reflecting film 2 is formed bysequential film-forming in the order of aluminum (Al), magnesiumfluoride (MgF₂) and titanium dioxide (TiO₂) by a vacuum evaporation onone surface 1 a of the cleaned glass substrate 1, and thereby areflecting mirror 3 is obtained. The thicknesses of the Al film, theMgF₂ film and the TiO₂ film are respectively within the range of 50 nmto 300 nm.

[0024] An explanation will be given on the function of the reflectingmirror 3 which is constructed as mentioned above. Parallel lightincident on the reflecting mirror 3 is reflected at a reflection angleequal to the incident angle.

[0025] Therefore, even if a glass substrate formed by a float process isused as the glass substrate 1 for the reflecting mirror 3, the lightreflected on the reflecting film 2 is prevented from going in differentdirections by forming the reflecting film 2 after the one surface 1 a ofthe glass substrate 1 is polished.

[0026] In a rear projection display according to the present invention,the above-mentioned reflecting mirror 3 is installed. As shown in FIG.2, the rear projection display comprises the reflecting mirror 3, anoptical unit 4, a screen 5 and so on. In such a device, the distancebetween the reflecting mirror 3 and the screen 5 is typically within arange of 20-70 cm.

[0027] The optical unit 4 comprises a high-brightness discharge lamp,three sheets of liquid crystal panel each of which corresponds to R(Red), G (Green) and B (Blue), a color filter and a projection lensbeing of a high caliber. Further, the screen 5 comprises a Fresnel lens,a lenticular to which a non-glare treatment is applied, and so on.

[0028] An explanation will be given of the function of the rearprojection display according to the present invention which isconstructed as mentioned above.

[0029] In the rear projection display, light (image) from the opticalunit 4 is reflected on the reflecting mirror 3 and projected on thescreen 5, thereby reproducing the image.

[0030] Even if a reflecting mirror 3, in which is used the glasssubstrate 1 formed by the float process, is installed as a mirror of therear projection display, by forming the reflecting film 2 after the onesurface 1 a of the glass substrate 1 is polished, it is possible toprevent any light and shade pattern caused by a difference in lightintensity caused by minute irregularities on the surface 1 a of theglass substrate formed by the float process from occurring in the lightimage projected on the screen 5.

[0031]FIG. 3 shows the results of measurement of the irregularities onthe surface of the glass substrate before and after polishing. A shows agraph of data of the measurement taken on an axis perpendicular to thetransfer direction of the float process before polishing. B shows agraph of data of the measurement taken on an axis parallel to thetransfer direction of the float process before polishing. On the otherhand, C shows a graph of data of the measurement taken on an axisperpendicular to the transfer direction of the float process afterpolishing. D shows a graph of data of the measurement taken on an axisparallel to the transfer direction of the float process after polishing.

[0032] As shown in A and B, striped irregularities parallel to thetransfer direction of the float process occur on the surface of theglass substrate. Further, C and D show that such irregularities can bereduced by polishing.

[0033] As the reflecting mirror 3, a back-surface-coated mirror can beused as well as the front-surface-coated mirror shown in FIG. 1. Also,the reflecting film can be formed of another material or materials toachieve a reflectance adequate to utilize an image reflected thereby,which is not limited to the aluminum reflecting mirror. Further, as themethod for forming the reflecting film, another method can be usedinstead of the evaporation or sputtering film-forming method.

[0034] The reflecting mirror 3 according to the present invention can beapplied to a rear projection display in which one sheet of liquidcrystal panel is used by hologram color filter (film of diffractiongrating) as well as a rear projection display which requires threesheets of liquid crystal panel each of which corresponds to R (Red), G(Green) and B (Blue).

[0035] As is fully explained in the above, according to the presentinvention, it is possible to reflect parallel light, which is incidenton the reflecting mirror, at a reflection angle equal to the incidentangle, and thereby the reflected light is prevented from going indifferent directions.

[0036] Further, according to the present invention, it is possible tosufficiently achieve image quality required in a case where thereflecting mirror is applied as a mirror of a rear projection display.

[0037] Further, according to the present invention, it is possible tosufficiently achieve image quality required in a case where thereflecting mirror is applied as a mirror of the rear projection display,even if the distance between the reflecting mirror and the screen isincreased.

[0038] Further, according to the present invention, it is possible toimprove the reflectance of the reflecting mirror.

[0039] Further, according to the present invention, it is possible toprevent any pattern of light and shade in an image projected on a screenwhich occurs due to the difference in light intensity, and thereby theimage quality can be improved.

What is claimed is:
 1. A reflecting mirror for use in a rear projectiondisplay and so on, comprising: a glass substrate having at least onepolished surface which is polished to have a predetermined surfacewaviness; and a reflecting film which is formed on said polished surfaceof said glass substrate.
 2. A reflecting mirror as defined in claim 1 ,wherein said glass substrate is produced by a float process to haveminute irregularities of 0.1 μm or less on the measurement conditionthat the cut-off value is 0.8 mm to 8 mm.
 3. A reflecting mirror asdefined in either of claim 1 and claim 2 , said predetermined surfacewaviness being 0.05 μm or less on the measurement condition that thecut-off value is 0.8 mm to 8 mm.
 4. A reflecting mirror as defined inany one of claim 1 through claim 3 , wherein said reflecting film isobtained by film-forming in order of aluminum (Al), magnesium fluoride(MgF₂) and titanium dioxide (TiO₂).
 5. A rear projection display,comprising: a reflecting mirror as defined in any one of claim 1 throughclaim 4 ; an optical unit; and a screen.